Agriculture Reference
In-Depth Information
2.2.8 Polymer Blends
Polymer blends are polymeric materials made by mixing or blending two or
more polymers to enhance the physical properties of each individual component.
Common polymer blends include PP-polycarbonate, polyvinyl chloride (PVC)-
acrylonitrile-butadiene-styrene (ABS) and PE-polytetrafluoroethylene. Polymer
blending is accomplished by distributing and dispersing a minor or secondary
polymer within a major polymer that serves as a matrix. The major component can
be thought of as the continuous phase and the minor components as the distributed
phase or dispersed phase in the form of droplets or filaments. When creating a polymer
blend, one must remember that the blend will probably be re-melted in subsequent
processing or shaping processes. For example, a rapidly cooled system, frozen as a
homogenous mixture, can separate into phases because of coalescence when re-heated.
For all practical purposes, such a blend cannot be processed. To avoid this problem,
compatibilisers (which are macromolecules) are used to ensure compatibility in the
boundary layers between the two phases.
Blending can also be considered to be mixing, which is subdivided into 'distributive'
and 'dispersive' mixing. Studies of the development of the morphology of polymer
blends have shown three competing mechanisms: distributive mixing, dispersive
mixing, and coalescence. The process begins if a thin tape of polymer is melted away
from a pellet. As the tape is stretched, surface tension causes it to rip and to form
into threads. These threads stretch and reduce in radius until surface tension becomes
sufficiently significant to lead to disturbances. These disturbances grow and cause the
threads to break down into small droplets.
2.2.9 Filled Polymers
Fillers are, in general, accepted as being materials that are intentionally placed in
polymers to make them stronger, lighter, electrically conductive or cheaper. Any
filler will affect the mechanical behaviour of a polymeric material. For example,
long fibres will make it stiffer but usually denser, whereas foaming will make it more
compliant and much lighter. Conversely, a filler such as calcium carbonate will decrease
the toughness of the polymer while making it considerably cheaper. Some fillers such as
glass fibres or glass spherules, will increase the strength of a polymer, whereas others
will decrease it. 'Reinforced plastics' refer to polymers (matrix) whose properties have
been enhanced by introducing reinforcement (fibres) of high stiffness and strength.
Such a material is called a 'fibre-reinforced polymer' or a 'fibre-reinforced composite'.
 
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